1. Field of the Invention
The present invention relates to a liquid crystal display device provided with a plurality of sub-picture element electrodes to which mutually different voltages are applied in one picture element region.
2. Description of the Related Art
Liquid crystal display devices have advantages over Cathode Ray Tube (CRT) in that they are thin, light, and capable of being driven by a low voltage with small power consumption. Accordingly, liquid crystal display devices are used for various electronic instruments such as televisions, notebook personal computers (PC), desktop computers, personal digital assistants (PDA), and mobile phones. In particular, a liquid crystal display device of an active matrix type provided with a thin film transistor (TFT) as a switching element per each picture element (sub-pixel) exhibits display characteristics as good as those of the CRT due to its high driven capabilities and is widely used, for desktop PCs and televisions for example, in the area where the CRT has been used.
Generally, a liquid crystal display device is configured of two transparent substrates and a liquid crystal filled in a space between these substrates. Picture element electrodes, TFTs, and the like are formed for each picture element on one substrate whereas color filters opposite to picture element electrodes and a common electrode common to each picture element are formed on the other substrate. Hereinafter, a substrate on which the picture electrodes and TFTs are formed is referred to as a TFT substrate and a substrate placed opposite to the TFT substrate is referred to as an opposite substrate. Moreover, a structure configured by filling a liquid crystal in a space between the TFT substrate and the opposite substrate is called a liquid crystal panel.
Twisted nematic (TN) liquid crystal display devices formed by filling the horizontally aligned liquid crystals (the liquid crystals with positive dielectric anisotropy) in a space between two substrates and twisting and aligning liquid crystal molecules have hitherto been widely used. However, the TN liquid crystal display devices are associated with drawbacks such as the poor viewing angle characteristics and large changes in contrasts and color tones when viewed from an oblique direction. Accordingly, multi-domain vertical alignment (MVA) liquid crystal display devices have been developed and put to practical use.
Incidentally, although the conventional MVA liquid crystal display devices exhibit favorable viewing angle characteristics compared to the TN liquid crystal display devices, a phenomenon where the screen appears whitish occurs when viewed from an oblique direction.
FIG. 1 is a diagram showing T-V (transmittance-applied voltage) characteristics when viewing a screen from the front and from the direction 60° above. In FIG. 1, the horizontal axis represents voltage applied, and the vertical axis represents transmittance. As shown in FIG. 1, when a voltage somewhat higher than the threshold voltage is applied to a picture element electrode (the area enclosed by a circle in the figure), transmittance when viewed from an oblique direction becomes higher than that when viewed from the front. In addition, when the applied voltage is enhanced to some extent, the transmittance when viewed from an oblique direction becomes lower than that when viewed from the front. Accordingly, differences in brightnesses among red, green, and blue picture elements becomes small when viewed from oblique directions and a phenomenon where the screen appears whitish occurs as described earlier. This phenomenon is called a “wash out”. Wash outs occur not only in MVA liquid crystal display devices but also in TN liquid crystal display devices.
A provision of a plurality of sub-picture element electrodes in one picture element and capacitively coupling these sub-picture element electrodes are proposed in the specification of U.S. Pat. No. 4,840,460. Since the voltage applied to each sub-picture element electrode is determined according to capacitances between each of sub-picture element electrodes in such liquid crystal display devices, it is possible to apply mutually different voltages to each sub-picture element electrode. Therefore, a plurality of regions with different threshold values for T-V characteristics are apparently present in one picture element region. When there is a plurality of regions with different threshold values for T-V characteristics in one picture element region as described so far, T-V characteristics of that picture element region will be the T-V characteristics of combined T-V characteristics of each sub-picture element region. As a result, a phenomenon where the transmittance when viewed from an oblique direction becomes higher than that when viewed from the front is suppressed, and thus the phenomenon where screen appears whitish (wash out) is also suppressed.
Moreover, a liquid crystal display device formed by dividing a picture element electrode into a plurality of sub-picture element electrodes and placing a control electrode below each sub-picture element electrode with an insulating film interposed in between the control electrode and each sub-picture element electrode in order to improve viewing angle characteristics is also disclosed in Japanese Patent Laid-open Official Gazette No. Hei. 5-66412. In this liquid crystal display device, the same voltage is applied to each control electrode via the TFT and the voltage in accordance with the capacitance between the control electrode and the sub-picture element electrode is applied to each sub-picture element electrode.
A method of improving displaying characteristics by dividing one picture element region into a plurality of sub-picture element regions with different T-V characteristics as described in these official gazettes is called the halftone grayscale (HT) method.
FIG. 2 is a schematic diagram showing an example of a conventional MVA liquid crystal display device adopting the HT method. One picture element is divided into a first sub-picture element region A1 and a second sub-picture element region A2 in this liquid crystal display device.
TFT (not shown), an insulating film 11, a control electrode 12a connected to a source electrode 12 of the TFT, a first sub-picture element electrode 13a, and a second sub-picture element electrode 13b are formed on a TFT substrate 10. The first sub-picture element electrode 13a is placed on the insulating film 11 in the first sub-picture element region A1 and is electrically connected to the source electrode 12 of the TFT via a contact hole. Moreover, the second sub-picture element electrode 13b is placed on the insulating film 11 in the second sub-picture element region A2 and is capacitively coupled with the control electrode 12a with the insulating film 11 interposed in between.
On the other hand, a common electrode 21 and projections for alignment control 22 are formed on the opposite substrate 20. The common electrode 21 is opposite to both sub-picture element electrodes 13a and 13b while interposing a liquid crystal layer in between. Moreover, the protrusions 22 are formed of a dielectric such as resins and placed almost in a central position between the first and second sub-picture element regions A1 and A2.
Hereinafter, a sub-picture element electrode directly (in other words, without via capacitive coupling) connected to the TFT like the sub-picture element electrode 13a is also referred to as a directly connected picture element electrode. In addition, a sub-picture element electrode connected to the TFT via capacitive coupling like the sub-picture element electrode 13b is referred to as a capacitively coupled picture element electrode.
FIG. 3 is a diagram showing an equivalent circuit of the liquid crystal display device shown in FIG. 2. In this FIG. 3, reference numerals 15 and 16 denote a gate bus line supplied with scanning signals and a data bus line supplied with display signals, respectively. Moreover, Cs is an auxiliary capacitance connected between the source electrode 12 of a TFT 17 and earth, and CLC1 is a capacitance between the sub-picture element electrode (directly connected picture element electrode) 13a and the common electrode 21. Furthermore, C1 is a capacitance between the control electrode 12a and the sub-picture element electrode (capacitively coupled picture element electrode) 13b, and CLC2 is a capacitance between the sub-picture element electrode 13b and the common electrode 21.
Liquid crystal molecules 30 in the first and second sub-picture element regions A1 and A2 are all aligned almost perpendicular to a substrate surface when no voltage is applied to the liquid crystal layer (initial state). It should be noted that the liquid crystal molecules in the vicinity of the protrusions 22 are aligned almost perpendicular to inclined planes of the protrusions 22.
When the scanning signals are supplied to the gate bus line 15 and the TFT 17 is turned on, the display signals are supplied from the data bus line 16 to the source electrode 12 and the liquid crystal molecules 30 are inclined at an angle according to the applied voltage. Since the liquid crystal molecules 30 are encouraged to align perpendicular to the inclined planes of the protrusions 22 at this time following the example of the liquid crystal molecules in the vicinity of the protrusions 22, directions in which liquid crystal molecules 30 are inclined are mutually different in both sides of the protrusions 22 and thus, multi-domains are achieved.
Moreover, a voltage of the display signals is applied to the sub-picture element electrode 13a whereas a voltage of the division of the display signals by C1 and CLC2 is applied to the sub-picture element electrode 13b in this liquid crystal display device. In other words, the voltage applied to the sub-picture element electrode 13b will be lower than that applied to the sub-picture element electrode 13a. Accordingly, as shown in FIG. 2, inclination angle (an angle inclined from that of the initial state) of liquid crystal molecules 30 in the second sub-picture element region A2 will be smaller than that of liquid crystal molecules 30 in the first sub-picture element region A1.
As described so far, favorable viewing angle characteristics can be obtained owing to the formation of a plurality of regions with mutually different alignment directions of liquid crystal molecules in one picture element in the MVA liquid crystal display device shown in FIG. 2.
However, inventors of the present invention and others consider that there are problems associated with the above described, conventional MVA liquid crystal display device as follows. That is, since the voltage applied to the liquid crystal layer in the second sub-picture element region A2 is lower than that applied to the liquid crystal layer in the first sub-picture element region A1, brightness in the white display mode will be lower than that of MVA liquid crystal display devices not adopting the HT method. When the applied voltage is further increased in order to enhance the brightness in the white display mode, the peak of T-V characteristics is exceeded in the first sub-picture element region A1 and the brightness is reduced, and also a phenomenon where the screen appears yellow occurs when viewed from the front.